1. Field of the Invention
This invention pertains to apparatus and a method for reducing pressure pulsations in liquid conduit systems connected to positive displacement pumps and the like.
2. Background Art
In the art of controlling pressure pulsations and vibrations resulting therefrom in hydraulic systems a number of devices have been developed which are adapted primarily to control pressure variations resulting from sudden changes in flow rate of fluid through the system. For example, in the discharge piping system connected to positive displacement pumps it is known to use pressure pulsation dampeners which are adapted primarily to reduce the time spaced pressure pulses resulting from flow variations caused, for example, by single and multicylinder reciprocating plunger pumps. The discharge flow stream from a reciprocating plunger type is not at a continuous rate due to the basic mechanism of reciprocating piston or plunger type pumps. Although multicylinder plunger pumps with three or more cylinders have been developed to provide for smoother and more continuous discharge flow, pressure pulsations resulting from flow variations which cannot be totally eliminated cause vibrations and disturbances in the piping system which can ultimately damage the discharge flow pipe or other components of the system as well as the pump itself.
In this regard a number of pressure pulsation dampening devices have been developed. One of the most common types of pressure pulsation control devices comprises a relatively small pressure vessel interposed in the pump discharge pipe as an appendage or branch conduit and being provided with a flexible gas filled bladder contained therein. The bladder contracts and expands in response to changes in pressure and flow in the pipeline and in the pressure vessel to reduce the flow variations and associated pressure pulsations. Pressure pulsation dampeners utilizing a pressure vessel partially filled with compressed gas and being in communication with the pump discharge line have also been used to reduce pressure pulsation induced vibrations in positive displacement pump systems. This type of pulsation dampener has been largely replaced by the elastic diaphragm or bladder type described above due to the fact that the liquid in the system tends to absorb the gas in the vessel and, depending on the type of liquid being pumped, a gas compatible with the liquid is not always readily available or easily used with such systems.
Efforts to control flow induced pressure pulsations in hydraulic systems which include positive displacement pumps have also lead to the development of the so called acoustical or reaction type pulsation dampeners. Such type of dampeners, however, are effective only when designed for a rather specific hydraulic system including pressure, flow, fluid density and physical characteristics of the piping system downstream of the pump. Accordingly, such types of pulsation dampeners must be tailored to specific pump operating conditions and lose their effectiveness when system parameters are subject to moderate or significant variations. Moreover, acoustic type pulsation dampeners as well as certain flexible bladder type dampeners, if not properly sized, can contribute to pressure losses in the system with which they are used.
Although a great deal of effort has been focused on reducing flow induced pressure pulsations in hydraulic systems using positive displacement pumps, it has been observed that in many hydraulic systems certain pressure pulsations persist in the flow conduit or piping downstream of the dampener even though high amplitude flow induced pulsations may be adequately attenuated. In some hydraulic systems these undampened or so called residual pulsations are not of an amplitude sufficient to cause damage to the pump or the pipe network downstream thereof. In fact, many residual pressure pulsations and related vibrations appear to be related to pump speed and flow variations inherent in the operation of positive displacement pumps. Although some of these residual or secondary pressure pulsations cannot be dampened by conventional diaphragm, bladder and acoustic type dampeners they are sometimes successfully dampened by the use of baffles in the flow path of the discharge liquid.
However, it has been observed that some types of hydraulic systems resist the attenuation of certain pressure pulsations of relatively high amplitude at frequencies which do not appear to be related to flow variations resulting from actual pump design or operating characteristics. It has been discovered in pursuing the present invention that hydraulic systems which include positive displacement pumps, particularly of the reciprocating plunger type, are subject to damaging vibrations caused by pressure pulsations which are created by the acceleration and deceleration (negative acceleration) of the liquid at the beginning and end of the delivery stroke of the pump plunger, respectively. Moreover, acceleration induced pressure pulsations are not reduced by increasing the number of pump cylinders and plungers and tend to become a greater factor in the overall pressure pulsation phenomena in multicylinder pumps and high speed pumps. Although these acceleration induced pressure pulsations have been observed to be particularly significant in hydraulic systems utilizing positive displacement reciprocating plunger pumps, it is contemplated that other types of positive displacement pumps such as rotary gear, vane and helical screw types as well as so called axial piston types may, under certain conditions, require attenuation of flow acceleration induced pressure pulsations.
Moreover, these so called acceleration induced pressure pulsations appear to be much more pronounced and difficult to control in hydraulic systems where at least a portion of the discharge "head" or pressure is due to a vertical column of liquid and not as a result of friction or throttling losses in a pipe network. In other words in installations where a constant static head is present at the pump discharge flange regardless of pump discharge flow rate the secondary or acceleration generated pressure pulsations are of a significant magnitude and largely resist attenuation by known types of pulsation control equipment. It has also been determined that pumps operating with high density fluids such as well drilling muds and coal or iron ore slurries are also subjected to more pronounced magnitudes of acceleration induced pressure pulsations, and that these pulsations are generally proportional to fluid density.